Very candidacy before processing in PageLoaderMatcherPolicy (#16678)

Fixes https://github.com/aspnet/AspNetCore/issues/13996

src/Mvc/Mvc.RazorPages/src/Infrastructure/PageLoaderMatcherPolicy.cs

@@ -65,6 +65,11 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure
 
             for (var i = 0; i < candidates.Count; i++)
             {
+                if (!candidates.IsValidCandidate(i))
+                {
+                    continue;
+                }
+
                 ref var candidate = ref candidates[i];
                 var endpoint = candidate.Endpoint;
 

src/Mvc/test/Mvc.FunctionalTests/RoutingDynamicTest.cs

@@ -6,6 +6,8 @@ using System.Net;
 using System.Net.Http;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
+using RoutingWebSite;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Mvc.FunctionalTests
@@ -14,12 +16,13 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
     {
         public RoutingDynamicTest(MvcTestFixture<RoutingWebSite.StartupForDynamic> fixture)
         {
-            var factory = fixture.Factories.FirstOrDefault() ?? fixture.WithWebHostBuilder(ConfigureWebHostBuilder);
-            Client = factory.CreateDefaultClient();
+            Factory = fixture.Factories.FirstOrDefault() ?? fixture.WithWebHostBuilder(ConfigureWebHostBuilder);
+            Client = Factory.CreateDefaultClient();
         }
 
         private static void ConfigureWebHostBuilder(IWebHostBuilder builder) => builder.UseStartup<RoutingWebSite.StartupForDynamic>();
 
+        public WebApplicationFactory<StartupForDynamic> Factory { get; }
         public HttpClient Client { get; }
 
         [Fact]
@@ -99,5 +102,39 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
             Assert.Equal("Hello from dynamic page: /DynamicPage", content);
         }
+
+        [Fact]
+        public async Task AppWithDynamicRouteAndMapRazorPages_CanRouteToRazorPage()
+        {
+            // Regression test for https://github.com/aspnet/AspNetCore/issues/13996
+            // Arrange
+            var client = Factory.WithWebHostBuilder(b => b.UseStartup<StartupForDynamicAndRazorPages>()).CreateDefaultClient();
+            var url = "/PageWithLinks";
+
+            // Act
+            var response = await client.GetAsync(url);
+
+            // Assert
+            var document = await response.GetHtmlDocumentAsync();
+            var editLink = document.RequiredQuerySelector("#editlink");
+            Assert.Equal("/Edit/10", editLink.GetAttribute("href"));
+        }
+
+        [Fact]
+        public async Task AppWithDynamicRouteAndMapRazorPages_CanRouteToDynamicController()
+        {
+            // Regression test for https://github.com/aspnet/AspNetCore/issues/13996
+            // Arrange
+            var client = Factory.WithWebHostBuilder(b => b.UseStartup<StartupForDynamicAndRazorPages>()).CreateDefaultClient();
+            var url = "/de/area%3Dadmin,controller%3Ddynamic,action%3Dindex";
+
+            // Act
+            var response = await client.GetAsync(url);
+
+            // Assert
+            await response.AssertStatusCodeAsync(HttpStatusCode.OK);
+            var content = await response.Content.ReadAsStringAsync();
+            Assert.StartsWith("Hello from dynamic controller", content);
+        }
     }
 }

src/Mvc/test/WebSites/RoutingWebSite/StartupForDynamicAndRazorPages.cs

@@ -0,0 +1,62 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Routing;
+using Microsoft.AspNetCore.Routing;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace RoutingWebSite
+{
+    // For by tests for a mix of dynamic routing + Razor Pages
+    public class StartupForDynamicAndRazorPages
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services
+                .AddMvc()
+                .SetCompatibilityVersion(CompatibilityVersion.Latest);
+
+            services.AddSingleton<Transformer>();
+
+            // Used by some controllers defined in this project.
+            services.Configure<RouteOptions>(options => options.ConstraintMap["slugify"] = typeof(SlugifyParameterTransformer));
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+            app.UseRouting();
+            app.UseEndpoints(endpoints =>
+            {
+                endpoints.MapRazorPages();
+                endpoints.MapDynamicControllerRoute<Transformer>("{language}/{**slug}");
+            });
+        }
+
+        private class Transformer : DynamicRouteValueTransformer
+        {
+            // Turns a format like `controller=Home,action=Index` into an RVD
+            public override ValueTask<RouteValueDictionary> TransformAsync(HttpContext httpContext, RouteValueDictionary values)
+            {
+                if (!(values["slug"] is string slug))
+                {
+                    return new ValueTask<RouteValueDictionary>(values);
+                }
+
+                var kvps = slug.Split(",");
+
+                var results = new RouteValueDictionary();
+                foreach (var kvp in kvps)
+                {
+                    var split = kvp.Split("=");
+                    results[split[0]] = split[1];
+                }
+
+                return new ValueTask<RouteValueDictionary>(results);
+            }
+        }
+    }
+}